1. Field of the Invention
This invention relates generally to sonar equipment and in particular to an acoustical energy absorbing baffle.
2. Description of Prior Art
Just after World War II, Mason disclosed that sound could be absorbed by the use of slotted plates or screens. As taught in U.S. Pat. No. 2,415,832 he used an oil and immersed his slotted plates therein. He provided for a backing resonator for this configuration of baffle which absorbs underwater sound. The physical mechanism that he used is that of absorbing acoustic energy by the frictional losses created by the motion of the oil through the slots in plates. In order for the acoustic resistance to be appropriate for the loss to be achieved, it is necessary to control the size of the holes or slots in the plates to a high degree of precision. For the loss to be appreciable, the plate or screen must be kept relatively rigid. Goodman recognized the importance of this and, as taught in U.S. Pat. No. 4,353,768, developed a method for constructing lightweight resistive screens. Denaro et al. in U.S. Pat. No. 4,450,544 also employed the resistive screen technology to absorb underwater sound. The screens have a plurality of restricted orifaces in them and the absorbing mechanism is still due to the frictional losses generated by the oil moving through the orifaces in the screens. This mechanism is not the physical mechanism that is used by the present invention. The present invention absorbs acoustic energy in a manner that differs significantly from the resistive screen/oil frictional losses as taught by Denaro. The disadvantage of using the resistive screen/oil frictional loss mechanism is that the screen must be kept rigid. At low frequencies, say, below 800 Hz, the screens cannot be kept rigid in most sonar systems. This is because the screens are kept rigid as a result of mass loading. The mass of most ships is sufficient to mass load the resistive screens adequately for frequencies above 1000 Hz. But at low frequencies, below 800 Hz, most ships are not massive enough to sufficiently keep the screen rigid. Hence the absorption of sound is limited to above 800 Hz for a baffle that uses the resistive screen/oil frictional loss mechanism. Before discussing the present invention, however, other prior art should first be discussed.
Acoustic window materials are discussed in U.S. Pat. No. 3,858,165. The teachings found therein are not necessarily pertinent to the function of the present invention. While it is true that that an acoustic window of the kind disclosed in U.S. Pat. No. 3,858,165 can be used in the present invention, it should be noted that for frequencies below 15 kHz, a 1/16th inch thick piece of steel or other metal may work just as well. Use of the acoustic window as disclosed in U.S. Pat. No. 3,858,165 may be worthwhile, however, if among the frequencies of sound that is intended for absorption by the present invention there are higher than 20 kHz sound waves.
As taught in U.S. Pat. No. 4,399,526 elastomeric materials may be used to absorb underwater sound. The principle disadvantages with these kinds of absorbers are that their performance changes appreciably with even small changes in their temperature. This is because these materials have physical properties such as their bulk modulus, shear modulus and corresponding loss factors that change as the temperature of the material changes from, say, 33.degree. F. to 55.degree. F. or from 55.degree. F. to 75.degree. F. The acoustic loss is usually dependent therefore, on the temperature of the environment in which the elastomer is placed. This severely restricts the use of this baffle. Some transducers are also limited in their effectiveness for the same reason.
Consider for example, the transducer discussed in UK Pat. No. 2,063,007. The transducer here has a transducer element backed by an elastomer such as neoprene or urethane. Both the neoprenes and the urethanes have shear moduli and corresponding loss factors that change appreciably with small changes in temperature. For this reason, the transducer effectiveness will vary, depending upon the temperature of the water in which it is placed.
A similar difficulty is found with the attenuating material found in U.S. Pat. No. 4,528,652. In this patent, the attenuating material comprises a low viscousity gel with a filler of heavy oxides or metal powder. A gel of silicone epoxy or rubber is the preferred material in this patent. Again, the attenuating material uses the hysteresis loss that occurs in the epoxy or rubber. Such a hysteresis loss is quite temperature dependent, at least for the range of ocean temperatures for the earth (0.degree. C.-25.degree. C.).
The present invention uses strands of wire immersed in oil and a complaint mass, all of which is housed in a rigid housing. Sound causes the complaint mass to resonate and the resonating motion of the compliant mass causes motion in the wire strands that are proximate to it. The motion of the wire strands through the oil causes some acoustic loss. The wire strands also rub against each other. This rubbing action causes a frictional loss. The oil is selected so that the oil used in the present invention has a viscousity and a frictional coefficient that does not change appreciably over the changes in temperature found in the ocean. For this reason, the acoustic absorption of the baffle does not change appreciably as a function of ocean water temperature changes. The present invention is therefore a baffle that will give relatively consistant absorption performance characteristics as it absorbs underwater sound from the ocean in which it is immersed.
It should also be noted that it is theoretically possible to construct a baffle of the present design that can operate at frequencies well below 800 Hz. This is because the frictional losses in the porous material (i.e. the metal strands of wire immersed in oil) are the direct result of the motion of the complaint mass. By constructing a baffle with a compliant mass that resonates below 800 Hz, it is possible to cause the porous material of strands of wire to absorb the acoustic energy. Since the present invention does not, therefore, require any mass loading for the frictional losses to occur, it is theoretically possible to construct a baffle that can be effective down to 10 Hz or less. (Of course, a baffle effective at this frequency would have to be proven through experiment, and there are few experimental facilities capable of testing baffles at this frequency). More practical baffles would be higher in their effective frequencies.
It should be noted that my other patents, U.S. Pat. Nos. 4,419,617; 4,349,615; 4,327,161; 4,179,627; and 4,126,149 all deal with energy conversion of one form or another. They do not, however, deal with the absorption of acoustic energy.
It is an object of the present invention to provide a baffle that absorbs underwater acoustic energy encountered in the ocean environment.
It is a further object of the present invention to provide an acoustic baffle that is absorptive throughout the ocean temperature range.
It is yet a further object of the present invention to provide a baffle that is effective over a wide range of frequencies.